Photon sensors can be implemented using an array of microcells containing avalanche photo diodes (APD). The APDs can be fabricated on a silicon wafer as a silicon photomultipliers (SiPM). In conventional silicon photo multiplier devices each individual APD can be connected to a readout network via a quenching resistor having typical values between 100 kΩ-1 MΩ. When a bias voltage applied to the SiPM is above breakdown, a detected photon generates an avalanche, the APD capacitance discharges to a breakdown voltage and the recharging current creates a signal.
SiPM technology can have an intrinsic dark count (i.e., response in the absence of light—typically from thermionic emissions), which can be due to crystal defects, impurities, and other anomalies. The distribution of defects among the individual microcells of an array can be non-uniform resulting in the possibility of a small number of microcells per device having a very high dark count generation rate.
Noisy microcells within the array can be located by measuring the photoluminescence of the SiPM under an applied voltage. Identified noisy microcells can be disconnected from the array by using laser pulses. Actual implementation of this method is very complicated and expensive. For these reasons the approach is not attractive for implementation in high-volume SiPM production.
Another approach to identify noisy microcells is to measure the dark count of each microcell and programmatically inhibit noisy ones. To implement this approach, each microcell needs to have an address line with a unique address. Additionally, the individual microcells are fabricated to include a static memory cell that can be used to disable or enable the microcell. An external controller is required to implement the calibration process.